1. Field of the Invention
This invention pertains to document handling, and more particularly to apparatus for transporting and stacking envelopes.
2. Description of the Prior Art
Various equipment has been developed to print documents, fold them, and stuff them into envelopes. Additional equipment prints addresses and bar codes on the envelopes in preparation for franking and mailing. Such equipment is highly automated and is capable of handling large volumes of documents at low cost.
A vital component of a document handling system is a stacking machine that edge stacks the stuffed and addressed envelopes. The stacking machines are usually designed with two or more horizontally moving belts. Completed envelopes fall onto the belts from an upstream station, such as an addressing machine, in generally horizontal but overlapped fashion. The belts carry the envelopes to an angled stop at the downstream end of the stacking machine. The leading edge of the first envelope of a batch slides up against the stop under the impetus of the driving friction between the belts and the envelope's trailing or bottom edge, and the stop prevents further downstream motion of the envelope. Subsequent envelopes slide up against the respective adjacent preceding envelopes to form an angled stack.
As more envelopes are added to the stack, the upstream envelopes tip slightly backwardly, i.e., in the upstream direction. Backward tipping is caused by the combination of the compressibility of the envelopes and the constant driving force on their bottom edges by the belts. Ultimately, the stack attains a length such that the upstream envelopes therein tip completely backwardly onto the belts. At that point, the envelopes become disorganized and operation of the stacking machine must halt.
Examples of prior envelope stacking machines may be seen in U.S. Pat. Nos. 2,554,578; 3,817,516; 5,064,185; and 5,137,415. In a typical commercially available stacking machine, four belts have respective upper runs that slide along a deck that is between approximately two and four feet long. About 300 envelopes can be stacked by the prior machine before failure in the form of backward tipping occurs.
In addition to the relatively small number of envelopes they can stack successfully, the prior stacking machines have two other deficiencies. First, the continuously moving belts scuff the bottom edges of the envelopes in the stack. Second, the weight of the stacked envelopes on the belts causes friction between the belts and their underlying support surface. That friction may be sufficient to stall the belt driving motor, so undesirably large motors are often incorporated into the stacking machines.
Thus, a need exists for improvements in envelope stacking machines.